Molecular characteristics and biological effects of dissolved organic matter leached from microplastics during sludge hydrothermal treatment

Previous knowledge of dissolved organic matter leached from microplastics (MP-DOM) was mainly based on the aquatic environment. The molecular characteristics and biological effects of MP-DOM in other environments have rarely been examined. In this work, FT-ICR-MS was applied to identify MP-DOM leached from sludge hydrothermal treatment (HTT) at different temperatures, and the plant effects and acute toxicity were investigated. The results showed that the molecular richness and diversity of MP-DOM increased with rising temperature, accompanied by molecular transformation in the meantime. The oxidation was crucial whereas the amide reactions mainly occurred at 180–220 oC. MP-DOM promoted root development of Brassica rapa (field mustard) by affecting the expression of genes and the effect was enhanced with rising temperature. Specifically, the lignin-like compounds in MP-DOM down-regulated Phenylpropanoids biosynthesis, while CHNO compounds up-regulated the nitrogen metabolism. Correlation analysis presented that alcohols/esters leached at 120–160 oC were responsible for the promotion of root, while glucopyranoside leached at 180–220 oC was vital for root development. However, MP-DOM produced at 220 oC showed the acute toxicity to luminous bacteria. Considering the further-treatment of sludge, the optimum HTT temperature could be controlled at 180 oC. This work provides novel insight into the environmental fate and eco-environmental effects of MP-DOM in sewage sludge. [Display omitted] •MPs is a non-negligible source of DOM in sewage sludge during HTT.•Oxidation reaction is key to MP-DOM transformation while amide reaction mainly occurred at relatively high-temperature stage.•MP-DOM could promote plant growth by affecting gene expression.•Alcohol, esters and glucopyranoside compounds in MP-DOM are key to promote plant growth.•180oC is the optimum HTT temperature to remove MPs and convert it into harmless active MP-DOM.